As an example of a lighting device for inspection used for purposes such as appearance inspection of products, as described in Patent Literature 1, a coaxial lighting device that makes an imaging direction coincide with a direction of lighting an inspection object can be cited. The coaxial lighting device is provided with: a light source that emits inspection light in a horizontal direction; and a half mirror that is provided at a tilt between the inspection object and an imaging device provided above the inspection object, and arranged so as to reflect the inspection light toward the inspection object as well as transmitting reflected light from the inspection object toward the imaging device.
Meanwhile, in recent years, making it possible to, from a captured image, detect feature points such as defects that are difficult to detect even when using a lighting device for inspection as described above has been demanded. More specifically, there are cases where inspection is difficult, such as a case where the shape of a product to be inspected is special or complicated, and therefore it is difficult to irradiate inspection light at sufficient intensity or light amount, a case where even if inspection light can be irradiated, the amount of reflected light from points other than a desired inspection point is too large, and a case where feature points such as defects are too small or even insensible, and therefore contrast is not easy to see.
For example, it is possible to, by using a diaphragm or the like to limit an irradiation range of inspection light only to an inspection object, reduce the amount of stray light including reflected light and scattered light from objects other than the inspection object to increase inspection accuracy. However, for example, in the case where the inspection object is small or even insensible, it is difficult to detect a change amount by which irradiated light is changed by feature points such as defects, and therefore the contrast between a defect and a normal part is more difficult to see.
Also, an inspection object may have a flat surface in some cases, or in other cases, it may have a slight convex or concave surface, and therefore one cause of unsuccessful defect detection is that tilt states of the central axes of irradiation solid angles at respective points on the inspection object are not appropriate for the inspection object. However, in the past, there has not been known a lighting device for inspection that can appropriately set tilt states of the central axes of irradiation solid angles at respective points on an inspection object, and the magnitudes of the irradiation solid angles to tilt states and values appropriate for the inspection object, respectively.
Accordingly, there has been demanded a lighting device for inspection that makes it possible to, through machine vision, detect defects even for a product of which a difference between a defect and a normal part does not easily appear in a captured image as described above.